In modern computer networks, a storage server can be used for many different purposes, such as to provide multiple users with access to shared data or to back up mission critical data. A file server is an example of a storage server which operates on behalf of one or more clients to store and manage shared files in a set of mass storage devices, such as magnetic or optical storage based disks or tapes. The mass storage devices are typically organized into one or more volumes of Redundant Array of Independent (or Inexpensive) Disks (RAID).
One mode in which a file server can be used is a network attached storage (NAS) mode. In a NAS mode, a file server can be implemented in the form of an appliance, called a filer, that attaches to a network, such as a local area network (LAN) or a corporate intranet. An example of such an appliance is any of the Filer products made by Network Appliance, Inc. in Sunnyvale, Calif. A storage server can also be employed in a storage area network (SAN), which is a highly efficient network of interconnected, shared storage devices. In a SAN, the storage server (which may be an appliance) provides a remote host with block-level access to stored data, whereas in a NAS configuration, the storage server provides clients with file-level access to stored data.
Some storage servers, such as certain Filers from Network Appliance, Inc. are capable of operating in either a NAS mode or a SAN mode, or even both modes at the same time. Such dual-use devices are sometimes referred to as “unified storage” devices. A storage server such as this may use any of various protocols to store and provide data, such as Hypertext Transport Protocol (HTTP), Network File System (NFS), Common Internet File system (CIFS), Internet SCSI (ISCSI), and/or Fibre Channel Protocol (FCP).
A simple example of a storage file server is illustrated in FIG. 1. A storage server located at the primary site is coupled locally to a set of mass storage devices 4, and to a set of clients 1 through a network 3, such as a local area network (LAN). Each of the clients 1 may be, for example, a conventional personal computer (PC), workstation, or the like. The mass storage devices 4 may be, for example, conventional magnetic disks, optical disks such as CD-ROM or DVD based storage, magneto-optical (MO) storage, or any other type of non-volatile storage devices suitable for storing large quantities of data.
The storage server 2 receives and responds to various read and write requests from the clients 1. In a system which handles large volumes of client requests, it may be impractical to save data modifications to the mass storage devices 4 every time a write request is received from a client 1. The reason is that disk accesses tend to take a relatively long time compared to other operations. Therefore, the storage server 2 may instead hold write requests in memory temporarily and only periodically save the modified data to the mass storage devices 4, such as every 5 to 10 seconds. The event of saving the modified data to the mass storage devices is called a “consistency point”. At a consistency point, the storage server saves any data that was modified by the write requests to its local mass storage devices 4.
In this approach, there is a small risk of a system failure occurring between consistency points, causing the loss of data modified after the last consistency point. Consequently, in at least one prior art solution, the storage server 2 includes a non-volatile random access memory (NVRAM) in which it maintains a log of write requests received from clients since the last consistency point. This log is referred to as the “NVLog”. The NVLog includes a separate entry for each write request received from a client 1 since the last consistency point. Each NVLog entry includes the data to be written according to the corresponding request. The NVLog is only used in the event of a failure, to recover data that would otherwise be lost. In the event of a failure, it may be possible to replay the NVLog to reconstruct the current state of stored data just prior to the failure. After each consistency point is completed, the NVLog is cleared and started anew.
FIG. 2 is a block diagram showing the architecture of a storage server 2 known in the prior art. The storage server 2 includes one or more processors 21 and a system memory 22 coupled to each other by a north bridge. The north bridge 28 is also coupled to a Peripheral Component Interconnect (PCI) bus 23. The north bridge 28 provides an interface between peripheral components on the PCI bus and the processors 21 and system memory 22.
Each processor 21 is a central processing unit of (CPU) of the storage server 2 and, thus, controls the overall operation of the storage server 2. In certain embodiments, a processor 21 accomplishes this by executing software stored in system memory 22. Such software may include the operating system 24 of the storage server 2. Each processor 21 may be, or may include, one or more programmable general-purpose or special-purpose microprocessors, digital signal processors (DSPs), programmable controllers, application specific integrated circuits (ASICs), programmable logic devices (PLDs), or the like, or a combination of such devices. The system memory 22 is a random access memory (RAM) which stores, among other things, the operating system 24 of the storage server 2, in which the techniques introduced herein can be implemented.
Connected to the PCI bus 23 are an NVRAM 29, which stores the NVLog of the storage server 2; one or more internal mass storage devices 25; a storage adapter 26; a network adapter 27; and a cluster interconnect adapter 30. Internal mass storage devices 25 may be or include any conventional medium for storing large volumes of data in a non-volatile manner, such as one or more disks. The storage adapter 26 allows the storage server 2 to access the external mass storage devices 4 and may be, for example, a Fibre Channel adapter or a SCSI adapter. The network adapter 27 provides the storage server 2 with the ability to communicate with remote devices such as the clients 1 over a network and may be, for example, an Ethernet adapter. The cluster interconnect adapter 30 provides the storage server 2 with the ability to connect the storage server to other storage servers in a cluster configuration.
One problem with the system described above, is that during consistency point processing, the NVlog may become full during heavy loads. If the NVlog is full, no further write requests can be acknowledged until the NVLog is cleared when the consistency point processing is completed. This adds latency to applications using the storage server.
For some applications, the added latency is of little concern. For example, employee home directories of an enterprise can slow down without any significant harm to the enterprise. However, for other applications, the added latency can cost the enterprise money. For example, a mission critical database, such as a customer service database, experiencing latency can hurt the enterprise in financial terms because the customer service representatives will handle fewer customers. What is needed is a file server that can differentiate between various applications when heavily loaded.